The Origin and Evolution of Mammals - Moodle

evidence to relate the haramiyidans to any other
mammal group.
Having stated this, there are other groups of peculiar
early mammals that may themselves be modified
haramiyidans. Sigogneau-Russell (1983; Sigogneau-
Russell et al. 1986) described some isolated teeth
from the Rhaetic of France. They have been named
Theroteinus, and consist of a broad occlusal surface
bearing two or three rows of low, rounded cusps
(Fig. 5.1(e)), and no sign of a propalinal component in
occlusion. The root was probably divided into two or
three, and the histology of the enamel resembles that
of other mammals. Sigogneau-Russell’s (1989a) tentative
suggestion was that they are the milk teeth of
haramiyidans, since normal teeth of the latter are
abundant at the locality. Butler (2000) agreed that
Theroteinus is a haramiyidan and suggested that it
was a form whose molar teeth had adapted for crushing
hard, brittle food. Kermack et al. (1998) described
Eleutherodon (Fig. 5.1(f)) on the basis of some isolated
Middle Jurassic teeth that resemble Theroteinus, but
J
MX
SMX
MX
PMX
SQ
N
F
P
P
N AL
Sinoconodon
ant. lam
have an extra row of cusps on the uppers and more
cusps on both. They also have wear patterns indicating
a propalinal action.
Whatever their relationships may be, haramiyidans
were certainly a very early, specialised
and short-lived, derivative from the hypothetical
mammalian ancestor that nevertheless underwent
a modest radiation.
Sinoconodon
THE MESOZOIC MAMMALS 141
Crompton and Sun (1985) described a complete
skull of the critically important Chinese mammal
Sinoconodon (Fig. 5.2), dating from the Lower Jurassic
Sinemurian Stage, about 200 Ma. It possesses a number
of characters suggesting that it is the most basal
form attributable to the Mammalia. Further specimens
available to Crompton and Luo (1993) amply
confirmed this conclusion, indicating that it is
something of a mosaic of primitive and derived features.
Confirmation of its mammalian status is
MX
L
MX
PAL
D
P
occ. cond.
Figure 5.2 Skull of Sinoconodon changchiawaensis. Skull length approx. 6 cm (Crompton and Sun 1985 and Crompton and Luo 1993). AL,
alisphenoid; ant.lam, anterior lamina; D, dentary; F, frontal; J, jugal; MX, maxilla; N, nasal; P, parietal; PMX, premaxilla; SMX, septomaxilla.